1. Field of the Invention
This invention relates to improvements in a method of producing a valve lifter to be used in an automotive internal combustion engine, and more particularly to the improvements for facilitating the production of the valve lifter while lowering the production cost of the valve lifter.
2. Description of the Prior Art
An automotive internal combustion engine of the DOHC (double overhead camshaft) type is provided with a valve operating mechanism including valve lifters of the directly operated type. Each valve lifter is disposed between a cam integrally formed on a camshaft and an engine valve such as an intake valve to transmit the lift of the cam to the intake valve.
The valve lifter is made of steel and formed generally cup-shaped or into the shape of a cylinder having an upper closed end. The valve lifter includes a cylindrical skirt section which is slidable inside a guide opening formed in a cylinder head. A disc-shaped crown section is formed integral with the inner peripheral surface of the upper part of the skirt section so as to serve as the upper closed end. The crown section is formed with a fitting depression whose periphery is defined by the upper part of the skirt section. A shim is fixed at the top of the skirt section in such a manner that the lower section thereof is press-fitted in the fitting groove. The outer peripheral surface of the cam is brought into slidable contact with the upper surface of the shim. The crown section is integrally provided at its back side surface (facing the intake valve) with a contacting projection or boss which is generally disc-shaped and has a small diameter, in which a valve stem of the intake valve is to be brought into contact with the contacting projection.
The valve lifter is usually produced according to a production process shown in FIG. 6. In this production process, first an iron-based base material is subjected to a cold forging to form a valve lifter-shaped base material having the contacting projection having a diameter of about 7 mm, at a step S1C. The valve lifter-shaped base material is subjected to an annealing and a bonderising at a step S2C. Then, a cold pressing is applied to the valve lifter-shaped base material to obtain a necessary hardness of the whole valve lifter-shaped base material, at a step S3C. Subsequently, the valve lifter-shaped base material is rinsed at a step S4C. Then, grinding is made on the valve lifter-shaped material to trim the outer shape of the skirt section at a step S5C. Further, machining is made on the whole body of the valve lifter-shaped base material including the upper surface of the crown section and the surface of the contacting projection so as to trim the shape of the valve lifter-shaped base material, at a step S6C. Thereafter, a cutting-out machining is made onto the upper surface of the crown section to form a fitting depression at the upper surface of the crown section, at a step S7C. A carburizing hardening is made onto the whole valve lifter-shaped base material to increase the hardness of the valve lifter-shaped base material, at a step S8C. Then, the outer surface of the valve lifter-shaped base material is subjected to finish-grinding in order to obtain an outer diameter precision after deformation of the outer surface due to the heat treatment, at a step S9C. The inner surface of the fitting depression and the surface of the contacting projection are subjected to finish-grinding to obtain a precision of the inner diameter and the like of the valve lifter-shaped base material, at steps S10C and S11C. Then, the whole body of the valve lifter-shaped base material is subjected to rinsing and preserving at a step S12C. Lastly, the valve lifter-shaped base material is subjected to inspection at a step S13C and forwarded at a step S14C.
However, drawbacks have been encountered in the above conventional production method of the valve lifter, in which grinding is again made to the whole body of the valve lifter after the heat treatment, thereby unavoidably increasing the number of steps of the production process of the valve lifter. This not only lowers the operation efficiency of the production but also increases the cost of production equipment so as to contribute to a rise in production cost.